This invention relates mainly to an apparatus for producing potable water together with ice slush from sea-water or brine, for instance on board small fishing boats, and relates particularly to such equipment where salt water is crystallized in a continuous process.
There are known methods of continuous liquid cooling to the freezing point already in use. Examples of such methods are found in U.S. Pat. Nos. 3,328,972 and 3,347,058. Generally, water is crystallized from a highly concentrated mixture. This is the basic method of making ice cream or producing frozen concentrated juices, for example.
In U.S. Pat. No. 4,557,159 and Canadian patent 1,208,027 the idea of water crystallization is applied to a low concentrate mixture, for example 3% of chlorides (salts) in water. Conventional methods of making ice commonly use a refrigeration process, wherein ice crystals are formed in a thick layer on an evaporator cooling wall from which they are constantly removed as, for example, flakes or chips. Auger type ice making machines and ice cream or high concentrated slush making machines commonly use this type of method. Other low concentrate mixtures methods, for example using sea water, form the fresh ice crystals inside the salt and water mixture rather than on the evaporator cooling wall(s). For example, two scouring blades and a cylindrical chamber having a diameter of about three inches and a scouring paddle rotation of about 350 rotation mer minute was found satisfactory to prevent ice crystal formation on the evaporator cooling wall(s). Thus, the time interval between the freezing temperature point of the brine and starting point of the ice crystallization (about one degree Celsius) could be carefully maintained to avoid evaporator xe2x80x9cfreeze upxe2x80x9d. A standard xe2x80x9cfloodedxe2x80x9d refrigeration circuit is described, in which a chamber surrounded by a jacket to which a condensed refrigerant is constantly supplied from condenser. The refrigerant boils in the jacket. It should be noted that at the idle conditions when the refrigeration compressor does not operate, the refrigerant, for example xe2x80x9cFreon-22xe2x80x9d, is expanded and the pressure inside the jacket reaches up to 160 pounds per square inch depending upon the outside temperature and evaporator insulation. A jacketed evaporator of more than 6 inches diameter having more than 15 pounds per square inch pressure is considered being a pressure vessel and must be manufactured and serviced as per ASME CODE. This increases the manufacturing costs and such an evaporator is not convenient to have on a small fishing boat.
FIG. 8 shows a phase diagram of the binary system water and xe2x80x9csaltxe2x80x9d and thus the relationship of the seawater concentration and the ice formation condition. The brine is mainly a water and NaCl mixture and is called as solute. The eutectic point D1 is a point where the three state conditions (phases) liquid, salt and ice exist side-by-side. It will be reached at the specific combination of temperature, concentration and pressure indicated. At a seawater or waste brine concentration of 2.8% salt, the liquid freezing point is about 29.3 degrees Fahrenheit (minus 1.5 degrees Centigrade, point B in FIG. 8). The process follows the line B-C1 when the temperature goes down, salt concentration increases and ice crystallization starts. Point C1 represents the following conditions: 4.0% salt concentration, 28.2 degrees Fahrenheit (minus 2.3 degrees Centigrade) and slush concentration by volume is 80/20 (80% brine to 20% ice crystals.
The phase transformation from liquid to solid (water to ice crystals) happens very suddenly. The brine or solute is cooled down below its freezing point B when the crystallization starts. The length of the time interval between the cooling and crystallization processes is critical and depends on many factors of the process, such as brine concentration, brine turbulence near the cooling surfaces, specific heat of coolant or refrigerant, overall heat transfer coefficient, and discharge rate of the slush that forms inside the evaporator. All these factors must be addressed and optimized to avoid xe2x80x9cfreeze-upxe2x80x9d and to provide a high efficiency apparatus.
The known ice cream machines and juice concentration freezers commonly use a high concentrate liquid, solute, with a low freezing point and uses a high specific heat refrigerant inside the evaporator jacket. A thick layer of ice crystals forms on the evaporator walls, which are constantly scoured by a low speed auger. The high concentrate liquid having a low freezing point gives a wider range of operating conditions, to avoid xe2x80x9cfreeze-upsxe2x80x9d, and a powerful auger drive provides enough force to scrape the thick layer of ice from the walls to a discharge opening. This type of refrigeration equipment can not be applied to low concentrate liquids, such as 3% brine or seawater. It is not economically feasible because of the high power consumption, non-efficient for the resulting low yield slush production and, most importantly, low concentrated brine too easily freezes up inside the evaporator.
U.S. Pat. No. 4,551,159 and Canadian patent 1,208,027 make an attempt to modify the ice making process for low concentrate brine and increase the overall heat transfer coefficient and as a result the evaporator capacity. A 3% brine/seawater concentration is used, and a required high turbulence of the brine near the cooling surfaces of the evaporator is provided. The cold layer of the brine is removed from the cooling surfaces before the ice crystals forms on them. An efficient heat transfer process is achieved, when the liquid refrigerant boils up inside the evaporator jacket. Unfortunately, the high heat transfer condition too easily causes xe2x80x9cfreeze-upxe2x80x9d problem when applied to 3% brine or seawater. In practice, this happens because of the fast cooling of the heat transfer surfaces. The ice crystal formation interval set at 1 degree between cooling and point of crystallization is a sensible parameter that requires special means to control and which is difficult to maintain, unless the salt concentration of the brine is increased to at least to 4% or a better regulated refrigeration system is used. The agitator rotation can partially solve the xe2x80x9cfreeze-upxe2x80x9d problem: the shorter the interval between consecutive wiping of the cooling surfaces, the faster removal of the cold layer will be that makes it possible to avoid ice formation on the cooling surfaces. U.S. Pat. No. 4,551,159 provides the specific calculations and suggests that a scraper rotation of 350 rotations per minute for a 3xe2x80x3 diameter evaporator is satisfactory to minimize xe2x80x9cfreeze-upxe2x80x9d problems. This solution is not practical for a larger diameter evaporator, for example a 12xe2x80x3 diameter evaporator, from a mechanical and structural point of view, because the linear velocity of the scrapers inside the evaporator increases up to 290 meters per second, for example in the case of a 12.5xe2x80x3 diameter evaporator. Canadian patent 1,208,027 mentions 150 wipes per minute scraper rotation.
U.S. Pat. No. 4,936,102 describes a method and apparatus for cooling down fish on a fishing boat. This patent suggests the idea to use seawater as make-up fluid to make more then 3% concentrated solution to produce ice xe2x80x9cslurryxe2x80x9d and eliminate xe2x80x9cfreeze-upxe2x80x9d problems. It is apparently not a very economical solution to have an additional salt storage and extra equipment and it is therefor not a practical solution for small fishing boats (usually low cost operations).
It is an object of the invention to improve on the existing ice slush production machines and provide a machine which facilitates the slush production from ordinary sea water by combining the slush production with desalination of the salt water, to also produce potable water, and utilizing the salt brine from the desalination process in the ice slush production.
In the invention, a water de-salination and ice slush producing system advantageously has the following components:
a submersible pump, for pumping seawater from the sea through pre-filtration stage filters,
a high-pressure pump, for transferring the seawater to at least one reverse osmosis membrane, where a separation of potable water and waste brine occurs,
a potable water storage tank, to which the potable water is transported,
a collecting tank, to which waste brine is transported, and
a chiller tube, for producing ice slush from the sea water mixed with waste brine.
The chiller tube of the invention has:
an outer housing,
a first cylindrical wall, which together with the outer housing defines a first cooling space with a first cooling conduit for circulating a coolant to cool down the first cylindrical wall,
a second cylindrical wall defining an ice generating space together with the first cylindrical wall, an inside of the second cylindrical wall defining a second cooling space having a second cooling conduit, wherein coolant enters the second cooling space via a coolant inlet, is cooled by the second cooling conduit having a refrigerant circulated inside from a refrigerant inlet to a refrigerant outlet, so that the coolant in the second cooling space cools the second cylindrical wall, and the coolant is then circulated through the first cooling conduit to cool the first cylindrical wall, and
a scraper assembly rotatably arranged inside the ice generating space, the scraper being rotated by a power source to remove fresh ice crystals from both an inner surface of the first cylindrical wall and an outer surface of the second cylindrical wall and transfer the ice crystals to an outlet in fluid communication with the ice generating space.
The outlet is advantageously arranged on a top of the chiller tube.
The chiller tube further advantageously has a top cover and a bottom cover, the top cover and the bottom cover being sealingly attached to the first cylindrical wall and the second cylindrical wall.
The top cover and the bottom cover are also advantageously sealingly attached to the outer housing.
Advantageously, the scraper assembly has at least two support rings having outer and inner blades for scraping ice slush.
The blades are advantageously raked for moving an ice slush flow upward to the outlet, for instance by having raked wings attached.
Further, the blades are advantageously spring loaded towards the respective surface to be scraped by biassing means, for example compression springs that press the edge of each blade towards the surface to be scraped.
An impeller is advantageously utilized to further enhance the ice slush flow in the desired direction inside the tube. The impeller is attached to the scraper, for instance, and thus propelled by the same power supply as the scraper.
Advantageously, all components of the system are mounted on a common frame, and also within the common frame, to conserve space and provide a compact and protected unit that is easy to transport.
Further features will be described or will become apparent in the course of the detailed description which follows.